Cook 6 slices chopped bacon; drain on paper towels. Pour off the excess fat and add 1 cup finely chopped onion and 1 finely chopped celery rib; cook until softened. Stir in 1/2 cup finely chopped apple, 1 finely chopped slice toasted sandwich bread and the bacon. Fill 16 white stuffer mushrooms. Bake in a 375° oven for 15 minutes.